In the simplest of animals, dissolved oxygen is directly absorbed from the surroundings, and carbon dioxide diffuses out. Animals like sponges, jellyfish, and flatworms absorb oxygen and release carbon dioxide in this way.

Direct gas exchange is possible only in animals that have internal organs close to the body surface. In a flatworm, no cell is more than a millimeter away from the water the creature lives in.

Some more sophisticated animals, like earthworms, use their whole body surface or skin for gas exchange.

Instead of exchanging gases directly with their environment, earthworms have a network of capillaries just under the skin through which gas exchange takes place. A blood-like fluid circulates throughout the body, transporting oxygen to cells and carrying away carbon dioxide.

Skin-breathers, like earthworms, must live in moist conditions or in water. In all animals, oxygen from the air must first be dissolved in water before it can be transferred to the body’s tissues.

Animals living outside an aqueous or moist environment require special adaptations that keep the respiratory surface moist.

Insects and other arthropods, such as spiders and centipedes, don’t have a network of blood vessels involved in gas exchange.

Instead, they have a system of tubes called tracheae that perform a similar function.

Tracheae branch inward from openings on the body surface called spiracles, through which air enters. Tracheae are ingrowths of the body wall, and they end in structures called tracheoles.

Tracheoles are filled with fluid, and the exchange of oxygen and carbon dioxide takes place by diffusion between the tracheoles and adjacent body cells. The extensive branching system of tracheae ensures that every body cell is close to a tracheole.

Gas exchange through tracheae and skin surfaces works well for small animals like insects and worms.

Larger organisms need a faster means of pumping oxygen - vertebrates have hearts that pump blood rapidly around the body. They need an efficient “fuel injection” system to provide large amounts of oxygen for their metabolism.

Most aquatic animals have structures called gills that are adapted for this purpose. Gills consist of slender filaments of epithelial tissue that contain branching capillaries.

Gas exchange occurs between the surrounding water and the blood in the gills. Fish take in water through the mouth and force it out over the gills.

In fish, blood and oxygen-rich water travel in opposite directions. This countercurrent flow ensures the maximum transfer of oxygen.

Gas exchange in land-dwelling vertebrates takes place in the lungs—chambers that are lined by moist epithelium with capillaries underneath.

In mammals, air enters the lungs through a single trachea. The trachea branches first into two bronchi, and then branches many times into smaller tubes called bronchioles. The smallest bronchioles end in alveoli—clusters of air sacs lined with a watery film.

Gas exchange occurs by diffusion between alveoli and networks of capillaries.

The development of gills for aquatic animals and lungs in terrestrial animals shows how two different structures have evolved to perform the same function – obtaining oxygen for cellular metabolism.

See if you can match the animals shown here with the way they carry out gas exchange.

Drag the labels into place. Then click Submit to see if you're correct.

Correct: Yes, that’s right!

All others: Sorry, that’s not quite right.

All: Simple multicellular organisms, such as jellyfish, don’t have respiratory structures. They exchange gases by direct diffusion between their cells and the surrounding water.

Some larger invertebrates, such as earthworms, exchange gases through the skin.

Insects and other arthropods, such as bees, have a system of tracheae that open to the exterior. Gas exchange takes place through these tubes.

Most marine and freshwater animals, such as fish, exchange gases through gills.

Land animals like reptiles, birds, and mammals exchange gases through lungs.

Now that we've looked at the different ways gas exchange takes place in different animals, we'll look in detail at the human respiratory system.

Copyright 2006 The Regents of the University of California and Monterey Institute for Technology and Education